The present invention relates generally to the field of frequency modification, and more specifically, to the field of multi-mode cellular telephone frequency conversion and modulation.
Multi-mode communication devices are capable of operating in two or more different modes for use in two or more different types of communication systems. Among various types of radios and radio communication devices, one type of multi-mode communication device is a dual-mode cellular telephone capable of operating in both analog FM (Frequency modulation) systems and digital CDMA (code division multiple access) systems.
CDMA/FM cellular telephones and other multi-mode communication devices typically include circuitry defining multiple signal paths for processing signals according to the multiple modes supported by the device. Normally, the various signal paths connect at some controlled switched point. It has been found that typical schemes for connecting the signal paths and switching between the various modes of communication can be improved. In one prior CDMA/FM cellular telephone scheme, the CDMA transmit signal path includes a CDMA IF (intermediate frequency) mixer section for up-converting a low frequency IF CDMA transmit signal into a higher frequency IF CDMA transmit signal. A separate FM transmit signal path includes an FM modulation section for modulating an intermediate carrier frequency with an FM audio signal to produce an FM IF signal. Subsequent to the FM modulation section and the CDMA IF mixer section, the two signal paths join each other in an adjustable amplifier section. Unfortunately, by keeping the signal paths separate until the IF stage, this connection scheme requires a large number of components, resulting in a larger, more expensive communication device.
There is, therefore, a need in the industry for a scheme for connecting signal paths of multi-mode communication devices which addresses these and other related, and unrelated, problems.